The present invention relates generally to filter molded heating and/or insulating bodies and more particularly to such bodies as may be employed as modular units in the construction of, and/or covers for high temperature furnaces.
Techniques for filter molding heating and/or insulating bodies are well-known in the art. One technique is described for example, in Hesse et al., U.S. Pat. No. 3,500,444. Briefly, the filter molding technique involves dispersing an inorganic refractory fiber and a suitable binder in a liquid suspension. A pressure differential is then created across a filter screen so that the liquid passes through the screen while the refractory members build up and accumulate a layer on the screen. After a suitable accumulation, the layer is removed and allowed to dry to form the body of insulating material. By placing a heating element against the screen prior to creating the differential pressure, the layer of insulating material can be built up about the element to form an electrical heating unit encapsulated with in an insulating refractory support. Smaller individual units formed in this manner may be stacked or otherwise assembled with similar units to form a larger heating unit.
One of the preferred binders utilized in such a method is colloidal silica. Where colloidal silica is used as the binder, it is known that upon drying of the filter formed body, there will be a movement or migration of the colloidal silica to the outer surface of the shape. This results in a case hardened exterior and a soft weak interior.
This case hardened exterior presents somewhat of a problem in modularized construction in that it is difficult to establish a seal good heat seal between the hardened surfaces of adjacent modules. In cases where insulating bodies are formed in this manner without the heating element, as may be used for example, as a furnace cover, the problem again exists of the hardened surface of the cover sealing effectively against the hardened surface of the furnace wall.
Simply placing a high temperature gasket, such as a ceramic fiber batting, between the adjacent surfaces will not completely solve the heat sealing problem because such gaskets are fairly delicate and difficult to position and handle. Another attempt of solving the gasket problem is shown in U.S. Pat. No. 3,786,162 wherein the adjacent surfaces of the modules are shaped complementary so as to mechanically engage each other.
In the present invention, a insulating body and/or heating element and its method of manufacture is provided wherein a layer of ceramic fiber blanket material is bonded to the surface of the insulating body by colloidal silica present at the surface for case hardening the body.